1. Field of the Invention
The present invention relates to a sheet conveying apparatus, an image forming apparatus, an image reading apparatus, and, more particularly, to an arrangement for correcting an advance and a delay of a sheet being conveyed.
2. Description of the Related Art
In a conventional image forming apparatus and image reading apparatus such as a copy machine, a printer, a scanner, and the like, a sheet conveying apparatus is attached to an image forming unit and an image reading unit to convey sheets such as recording materials, originals, and the like. The sheet conveying apparatus may be provided with a registration correction unit for correcting an advance and a delay of a sheet being conveyed so that the sheet reaches to the image forming unit and the image reading unit at a predetermined timing and with a skew feeding correction unit for aligning the attitude of the sheet.
There may be provided a detection unit as the registration correction unit to recognize a sheet conveying speed and a sheet position. Then, there is such a proposal for causing a sheet to reach to a predetermined position at a predetermined timing by controlling the sheet conveying speed of a conveying roller and a belt based on the signal from the detection unit.
For example, in an original feeder serving as the sheet conveying apparatus disposed to the image reading apparatus, after documents stacked on an original tray are fed by a pick-up roller, they are separated one by one in a separation unit and then conveyed to a reading unit by a conveying roller.
When an original is conveyed as described above, a registration correction unit disposed to the original feeder first predicts a time at which the original reaches to an image reading unit by recognizing a sheet conveying speed and a passing-through time by a conveying delay amount measurement sensor and an original extreme end detection sensor which come into contact with the original.
Then, the sheet conveying speed of the conveying roller is increased or decreased to correct the difference between the predicted reach time and a regular reach time so that the original reaches to the image reading unit at a predetermined timing by controlling the sheet conveying speed as described above (refer to, for example, Japanese Patent Application Laid-Open No. 2000-143036).
There are several systems serving as a skew feeding correction system of the skew feeding correction unit. One of the systems is arranged such that the extreme end of a sheet is abutted against the nip of a pair of rollers at rest and flexed so that the skew feeding of the sheet is corrected by causing the extreme end of the sheet to be aligned with the roller nip by the elasticity of the sheet. Further, when the roller is rotated at the timing at which a toner image on an image bearing member is transferred to a proper position of the sheet, a registration can be also performed in addition to the skew feeding.
Note that there is also another system arranged such that a shutter member for stopping the extreme end of a sheet is disposed at a midpoint of a sheet conveying path so as to be evacuated, and after the extreme end of the sheet is aligned with the shutter member, it is evacuated from the sheet conveying path to thereby correct skew feeding.
However, in these systems, the interval between sheets, that is, a so-called sheet distance is increased because it is necessary to stop the sheet once to correct the registration and the skew feeding, and thus the throughput during image formation and the like is lowered.
To cope with this problem, there is recentl proposed an active skew feeding correction system for correcting skew feeding while conveying a sheet without stopping it once to increase the throughput of image formation and the like image (refer to, for example, Japanese Patent Application Laid-Open No. 4-277151).
In the active skew feeding correction system, two sensors are disposed in a sheet conveying path so as to be separated form each other a predetermined distance in a width direction orthogonal to a sheet conveying direction and detect the inclination (skew feeding) of the extreme end of a sheet based on a signal illustrating that the extreme end of the sheet traverses the respective sensors. Thereafter, the skew feeding of the sheet is corrected by controlling sheet conveying speed of skew feeding correction rollers disposed in the width direction at a predetermined interval and driven independently.
However, in the sheet conveying apparatus provided with the conventional registration correction unit and the conventional skew feeding correction unit, a conveying roller and a skew feeding correction roller may be decentered by an assembly accuracy and variation per hour. When the conveying roller and the skew feeding correction roller are decentered as described above, the distance from the center of revolution and a sheet contact surface of the rollers is varied, thereby the sheet conveying speed of the conveying roller is changed at the cycle of the skew feeding roller when it is rotated once even if it rotates at a predetermined angular speed as illustrated in FIG. 9. That is, when the conveying roller and the skew feeding correction roller are decentered, they rotate irregularly and thus a sheet is conveyed at an irregular conveying speed.
Since a correction distance is the product of a sheet conveying speed V2 and a time T, even if a time T1 and time T2 illustrated in FIG. 9 are equal to each other, the areas (correction distances) in the two times T1, T2 are different from each other. Accordingly, when various types of corrections are performed by increasing or decreasing the number of revolutions of the roller, even if the angular speed, that is, the number of revolutions of a drive system is changed, a sheet is affected by the amount of dispersion of the sheet conveying speed due to the position at which the correction roller comes into contact with the sheet, and the amount of correction is also dispersed at the ratio.
When, for example, the skew feeding of a sheet is corrected by two skew feeding correction rollers, it is assumed that a skew feeding correction roller 300 illustrated in FIG. 10 has a diameter of 40 mm, a sheet conveying speed of 500 mm/second, and a correction time of 0.1 second. Further, it is assumed that the deviation of a conveying surface is 0.2 mm with respect to a roller axis 301, that is, the surface of the roller is deviated from the center of the roller axis 301 by 0.1 mm at the maximum.
In order to correct a skew feeding amount of 5 mm by one of the skew feeding correction rollers in this state, that is, in order to cause a delayed side to catch up to a not delayed side in the correction time of 0.1 second, the sheet conveying speed of the one skew feeding correction roller 300 must be increased by 50 mm/second (=5 mm÷0.1 second). That is, the skew feeding correction roller 300 on the delayed side must be driven so that the conveying speed thereof is increased to 550 mm/second.
Here, the angular speed of the skew feeding correction roller 300 at the speed of 550 mm/second is 1575.6°/second (550 mm/second÷(40 mm×π)×360°=1575.6°/second. Accordingly, the amount of revolution of the skew feeding correction roller 300 in the actual correction time of 0.1 second is 157.56° (1575.6°/second ×0.1 second=157.56°).
At this time, the sheet conveying speed in the vicinity of a position A (position farthest from the roller axis 301) of the skew feeding correction roller 300 and the sheet conveying speed in the vicinity of a position B position (position nearest from the roller axis 301) of the skew feeding correction roller 300 are as illustrated below.
When the speed is fastest (in the vicinity of the position A): 55.195 mm When the speed is slowest (in the vicinity of the position B): 54.8026 mm Then, the difference therebetween is as follows as illustrated in a part (a) of FIG. 10.ΔL=55.195−54.8026=0.3924 mm
Note that when the skew feeding correction roller 300 is a not decentered skew feeding correction roller as illustrated in a part (b) of FIG. 10, the difference between the sheet conveying speeds due to the difference of the phases of the roller is 0 mm. That is, when the skew feeding correction roller 300 is decentered, there is a possibility that an unexpected deviation of 0.3924 mm may occur at the maximum when the skew feeding of 5 mm is corrected. This value cannot be neglected in view of the positional accuracy when a formed image is allocated to a sheet.
Note that although the position of a sheet may be continuously detected by a line sensor and the like to eliminate the adverse effect due to the decentering of the skew feeding correction roller 300, a cost is increased in this case.
The present invention was made in view of the above circumstances and provides a sheet conveying apparatus, an image forming apparatus, and an image reading apparatus capable of stably correcting a delay or an advance of a sheet even if sheet conveying rollers such as a skew feeding correction roller and the like are decentered.